Because a superconducting accelerator cavity accelerates elementary particles that pass through the interior thereof, the characteristics of the inner surfaces thereof are important in order for a superconducting accelerator cavity to achieve a certain level of performance.
A superconducting accelerator cavity is formed by joining a plurality of ring-shaped members, such as cells, beam pipes, and so forth, which are placed one after another in an axial direction (see Patent Literature 1 and Patent Literature 2).
This joining is conventionally performed from outside by means of penetration welding in the form of electron beam welding or laser welding in a vacuum atmosphere because a vacuum atmosphere has low contamination caused by impurities.
In the case in which penetration welding from outside is employed in this way, unexpected irregularities sometimes occur in the welding condition inside the superconducting accelerator cavity, and post-processing thereof is time-consuming.
Because of this, as disclosed in Patent Literature 1 and Patent Literature 2, the welding work is typically performed by placing the plurality of ring-shaped members one after another in the axial direction so as to be horizontally oriented and by joining them so that joints are oriented in a vertical direction. By doing so, weld beads can be made substantially uniform on both sides of the joints, thus making it possible to form relatively smooth weld beads.